Active counter-vibration devices have been used in helicopters to oppose and cancel high levels of vibration transmitted from the rotor to the fuselage. In U.S. Pat. No. 4,819,182, electrohydraulic servoactuators are arranged in parallel with resilient mountings between the rotors and the fuselage, and act to vibrate the gear box inertially to generate cancelling forces.
However, in other helicopters, it may be impractical to incorporate resilient mountings between the rotors and the fuselage. In such instances, it may be necessary to employ mass elements that can be selectively vibrated to generate the desired counter-vibrations. These mass elements may be "parasitic" in the sense that they are not used for anything other than to produce such counter-vibration. Obviously, it is important that the added weight of such elements be kept to a minimum, while being able to produce the required levels of vibratory force.
The actively-controlled generation of counter-vibration forces by means of inertial reaction is addressed in Applicants' copending U.S. patent application Ser. No. 08/052,474, filed Nov. 23, 1993. In the apparatus disclosed therein, servo-driven mass-spring systems are operated at or near resonant frequencies to produce large mass motions, and hence large forces, with relatively small weight. However, the force-generating capabilities of such apparatus may be limited by practical limits of the spring stress created during such large-amplitude mass motions.
Higher mass acceleration levels can practicably be achieved by rotating a given mass eccentrically at the desired vibrational frequency, to produce a rotating force vector. It is known to use two counter-rotating eccentric masses to produce a linearly-oscillating force, similar to that developed by a mass vibrating on a spring, by summing two rotating vectors. The orthogonal components of these vectors add along a line of action, and cancel at right angles to it. The amplitude control necessary for counter-vibration applications requires that the oscillatory force produced by a second pair of counter-rotating eccentric masses be vector-summed with the similar oscillatory force produced by a first pair. The combined vibratory force amplitude can be adjusted from zero to four times the force produced by a single rotor by means of variable relative phase control. A mechanical implementation of such a device is disclosed in U.S. Pat. No. 3,208,292, in which two pairs of counter-rotating eccentric masses are coupled through an adjustable differential gearing arrangement to a common drive motor. This gearing is used to vary the phase relationship between the individual forces produced by the mass pairs, to control the amplitude of the resultant vector-summed force.
A similar device using a cluster of eccentric rotors, each driven by a separate electric servomotor, is disclosed in U.S. Pat. No. 5,005,439. This device produces an oscillating force vector in a plane which can not only be controlled in amplitude, but which can also be controlled in direction. It takes advantage of motor rotation angle control, combined with a nested mechanical packaging arrangement, to produce a device having controllable oscillatory force output. However, this device appears to be complex, bulky and awkward to mount on a structure.
Applicants' U.S. Pat. No. 5,347,884 also discloses rotating eccentric masses driven by independent electric servomotors with absolute rotary angle control. However, this invention utilizes a plurality of co-rotating eccentric masses to generate a rotating force vector, and uses a rotating couple in order to cancel the effect of a rotational unbalance disturbance on a structure. The significance of this disclosure is to illustrate the advantage of separately-controlled servomotor-driven motors to permit mounting an array of rotating vector devices at effective locations on a structure. See also, European Patent Application No. 88 400 904.4, published Oct. 18, 1989.